Method and device for measuring the thermal conductivity of a multifunctional fluid

ABSTRACT

The invention relates to a method and a device for the continuous measurement ( 30 ) of the thermal conductivity of a multifunctional fluid. The inventive method consists of: placing a sample of the multifunctional fluid in a space ( 31 ) which is defined by an inlet face and an outlet face; transmitting at least one very brief pulse of a heat flux to the sample via the inlet face, using a laser ( 40 ); measuring the heat wave at least three points which are spaced out inside the sample; using at least three temperature sensors (S 1 , S 2 , S 3 ) in order to determine the change in the temperature of the multifunctional fluid as a function of time at the three spaced-out points inside the sample; deducing the thermodynamic characteristics of the sample from the aforementioned temperature change and calculating the thermal conductivity from equation (I), wherein T represents thermal conductivity which is dependent on temperature, t represents thermal diffusivity which is dependent on k and which is equal to k(T)/ρ*Cp, ρ and Cp representing mass density and specific heat.

TECHNICAL FIELD

The present invention concerns a method for the continuous measurementof the thermal conductivity of a multi-functional fluid in which asample of the multi-functional fluid is passed through a space delimitedby a first face, called the entry, and a second face, called the exit,and in which an increase in temperature of the sample ofmulti-functional fluid is generated and this increase in temperature ismeasured.

It also concerns a device for the continuous measurement of thermalconductivity of a multi-functional fluid consisting of means for passinga sample of the multi-functional fluid through a space delimited by afirst face, called entry, and a second face, called exit, of the sample,means of heating to vary the temperature of this sample and meansdesigned to measure the variation of this temperature.

EARLIER TECHNIQUE

A multi-functional fluid is a fluid which can be comprised of severalcomponents which can be in different phases, liquid, solid or gaseous. Asimple example of a multi-functional fluid is blood. Othermulti-functional fluids are, for example, biphasic mixtures consistingof phase change materials, currently called PCMs, in suspension in aliquid and an ice slurry.

In order to be able to resolve the various problems of heat transfer,fluid flow or other, the numerical values of the physical andthermo-physical properties of fluids are of great importance.

Thermal conductivity in particular defines the degree of propagation ofheat in a material as a function of the temperature gradient.Conductivity is essentially a transfer of energy under the effect ofmovement, notably the vibrations of particles. The coefficient ofconductivity k (W/m.K) is dependent on the crystalline structure ofsolids, on the homogeneity, temperature, pressure, of the liquid, solidor gaseous phases and/or the composition.

It is noted that liquids are better conductors than gases, and solidsare better conductors than liquids. The conductivity of liquids dependsin the first instance on their temperature.

The precise measurement of the coefficient of conductivity is adifficult operation. In fact, the materials which are presently used arenot always similar. This leads to differences in the experimentalresults established by different research laboratories. Thus, theprecision related to the coefficient of conductivity does not exceed 5%.

For simple fluids, without a phase change, methods for the measurementof thermal conductivity already exist.

In order to characterize a multi-functional fluid with or without achange of phase, practically no direct, reliable method of measuringthermal conductivity exists.

The German publication DE 199 49 327 A1 describes a method and a devicefor the implementation of this method for determining the concentrationof a gas in a gaseous mixture comprised of several components. Themethod is based on the measurement of thermal conductivity of a gaseousmixture which is subjected to an increase in temperature between aminimum and maximum value determined by a temperature/time function. Ananalysis of the curve of temperature variation as a function of timepermits the determination of the concentration of a gas contained in themixture. The device includes a temperature sensor which transmits asignal to a Fourier analyzer. Such a device is not adapted to themeasurement of thermal conductivity of a multi-functional fluid.

DESCRIPTION OF THE INVENTION

The objective of the present invention is to alleviate this problem byproviding a method as well as a device which enables the determinationin a rapid, effective and economical manner of the thermodynamiccharacteristics of a multi-functional fluid, and to deduce the thermalconductivity there from.

This objective is attained by a method as defined in the preamble, andcharacterized by the facts that:

-   -   through the sample, through the first input face, at least one        very brief impulse of heat flux is transmitted,    -   the temperature is measured at least three separate points        within this sample,    -   by means of this measurement, the evolution of the temperature        of the multi-functional fluid is measured at these three points        as a function of time,    -   as a function of this evolution, the thermodynamic        characteristics of the sample of the multi-functional fluid is        determined, and the thermal conductivity of this sample is        determined.

According to one preferred method of implementation, the impulses ofheat flux are transmitted in a repetitive manner and a thermogram isestablished which consists of curves of the temperature evolution as afunction of time passing between the sending of a heat flux through thefirst input face and the increase in temperature determined at the atleast three separated points within the sample.

By preference, the thermal conductivity is deduced from the followingequation:${\frac{\partial T}{\partial t} + {{\alpha(k)}\left\lbrack {{{\frac{1}{k} \cdot \frac{\mathbb{d}k}{\mathbb{d}t}}\left( \frac{\partial T}{\partial x} \right)^{2}} + \frac{\partial^{2}T}{\partial x^{2}}} \right\rbrack}} = 0$

-   -   where: T is the temperature    -   k is the thermal conductivity dependent upon the temperature    -   t is the time    -   á is the thermal diffusivity dependant upon k and which is equal        to:    -   k(T)ρ*Cp    -   with ρ and Cp being the volume mass and the specific heat.

This objective is also attained by the device as defined in the preambleand characterized in that it consists, among other things, of meansdesigned to transmit to the sample, through the first input face, atleast a very brief impulse of heat flux, means designed to measure theheat wave at least three separated points within this sample, meansdesigned to determine on the basis of the measured values, the evolutionof the temperature of the multi-functional fluid as a function of timeat the separated points within the sample, means designed to deduce fromthis evolution the thermodynamic characteristics of the sample of themulti-functional fluid and means designed to calculate the thermalconductivity of this sample.

According to one preferred method of implementation, the means designedto pass a sample of the multi-functional fluid through the spacedelimited by the first and second faces includes an enclosure with aninsulating lining and an interior coating of polished metal, throughwhich is continually passed the multi-functional fluid.

The means (are) designed to transmit to the sample at least one verybrief impulse of heat flux comprised of at least one laser.

According to one particular preferred method of implementation, themeans designed to transmit to the sample at least one very brief impulseof heat flux can be comprised of an emitter tube.

The means designed to measure the heat wave which has passed through thesample is comprised preferably of a receiver tube.

According to one particularly advantageous construction, the meansdesigned to determine the evolution of the temperature of themulti-functional fluid as a function of time is comprised of at leastthree temperature probes designed to measure the temperature of thesample of multi-functional fluid at the at least three points.

The means designed to deduce, from the evolution of the temperature atthe three separated points in the sample of the multi-functional fluid,the thermodynamic characteristics of this sample and to calculate itsthermal conductivity, preferably comprised of an arithmetic unitdesigned to receive from the temperature probes signals corresponding tothe values measured.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention and its advantages will become more apparent inthe following description of the various modes of implementation of theinvention, by making reference to the appended drawings, in which:

FIG. 1 is a sketch of the principle illustrating the application of themethod according to the invention,

FIG. 2 is a view illustrating schematically a mode of implementation ofthe invention device,

FIG. 3 is a section view of an advantageous mode of implementation ofthe invention device, and

FIG. 4 represents a cross sectional view of a measuring probe used inthe invention device.

BEST METHOD OF IMPLEMENTING THE INVENTION

With reference to FIG. 1, the method consists firstly of selecting asample 10 of a multi-functional fluid to be studied, for example, byhaving it circulate between two linings 11 and 12 which are thermallyinsulated from a conduit or an enclosure of a form appropriate to definea first face, called input face, 13 and a second face, called exit face,14. The fluid is preferable subjected to an increase in temperature byconventional means. In addition, at least one very brief impulse of heatflux is transmitted across the first input face 13, illustrated by thearrow 15, for example, by means of a laser. Following this impulse, aheat wave propagates across the sample 10 and crosses the second exitface 14. It is represented by arrow 16 and measured by a device 17. Atleast three separate probes S1, S2 and S3 within the sample permit thetracing of the temperature evolution curve of the multi-functional fluidas a function of time by providing a thermogram. An arithmetic unitenables the deduction from this evolution of the thermodynamiccharacteristics of the sample of the multi-functional fluid, and thecalculation of the thermal conductivity of this sample. The methodpreferably includes the repeated emission of heat flashes and themeasurement is conducted in a repetitive manner.

Device 20 for the implementation of the method of measuring the thermalconductivity of a sample of a multi-functional fluid, illustrated by wayof a non-limiting example, in the form of an advantageous implementationby FIG. 2, consists of a first emitter tube 21 and a second receivertube 22, set up in such a way that the space separating their respectiveextremities 21 a and 22 a define the first input face 23 and the secondoutput face 24 of this sample. An impulse, called a flash of heat flux,is emitted by the emitter tube 21, crosses the sample in the form of aheat wave and is captured by the receiver tube 22. The two tubes areadvantageously several centimeters in length and have a diameter of lessthan 0,01 m. They contain the electronic components required to controlthe impulses and manage the measurements. They are mounted respectivelyon two supports 21 b and 22 b comprised of rigid conducting wires.

FIG. 3 is a cross sectional view of a measuring device 30 according tothe invention. It is mainly comprised of an enclosure 31 with aninsulating lining 32 and an interior coating of polished metal 33. Thisenclosure is traversed continually by a multifunctional fluid, such asfor example an ice slurry for which we wish to know the thermalconductivity. This fluid enters enclosure 31 by means of a conduit 34and leaves this enclosure by a conduit 35. It is in addition equippedwith a chamber 36 containing heating elements 37 which are designed tovary the temperature of the sample of multi-functional fluid. Inaddition, impulses of heat flux, represented by an arrow 38, aregenerated preferably in a repetitive manner, across the input face, forexample, by a laser 40. The heat waves generated traverse the sample offluid contained in the enclosure 31, exiting from the enclosure (arrow39) and are measured by at least three temperature probes S1, S2 and S3separated from one another and located within the sample. The thicknesse of the enclosure 31 is known precisely. This thickness can be variableto enable variation of the measurement parameters. To this end, device30 is equipped with instrumentation (not shown) comprised of amicrometer which allows the precise determination of the thickness e ofthe enclosure 31. The two conduits 34 and 35 are respectively equippedwith a valve 41, 42 which allows continuous control of the input, exitand circulation of the multi-functional fluid in the enclosure.

The probe 50, schematically represented by FIG. 4, corresponds to anadvantageous form of implementation of the temperature probes S1, S2 andS3 mentioned above. In fact, it combines the measurement of thetemperature and the measurement of the electrical conductivity. It isimmersed in the multi-functional fluid 51. It is comprised of atemperature sensor 52 and an electrical conductivity measurement sensor53 of the multi-functional fluid. These two sensors are for example,mounted on the interior lining of a tubular element 54 carried by asupport 55 which is immersed in the multi-functional fluid.

The device according to the invention functions advantageously in thefollowing manner. The means, for example the enclosure 31, permits theinsulation of a sample of the multi-functional fluid. The means,consisting of, example, instrumentation comprising a micrometer, enablesthe determination of the thickness of the enclosure. The means, forexample, consisting of heating elements 37, enabling the generation andraising of the sample temperature. The means such as the laser 40enabling the generation and transmission through the sample of at leastone very brief impulse of heat flux and preferably, a series of suchimpulses. The means such as the receiver tube 22, illustrated in FIG. 2,enabling the measurement of the heat wave which has traversed thesample. The temperature sensor 52 of FIG. 4 allows the determination ofthe temperature evolution of the multi-functional fluid as a function oftime. An arithmetic unit (not shown) enables the deduction from thisevolution of the thermodynamic characteristics of the sample of thefluid, and the calculation of the thermal conductivity of this sample.

To determine the thermal conductivity, it is advisable to solve the heatequation by considering that thermal conductivity is a function which isdependent on the temperature. This equation is the following:${\frac{\partial T}{\partial t} + {{\alpha(k)}\left\lbrack {{{\frac{1}{k} \cdot \frac{\mathbb{d}k}{\mathbb{d}T}}\left( \frac{\partial T}{\partial x} \right)^{2}} + \frac{\partial^{2}T}{\partial x^{2}}} \right\rbrack}} = 0$

-   -   where: T is the temperature    -   k is the thermal conductivity dependent on the temperature    -   t is the time    -   a is the thermal diffusivity dependent on k, and equals:        k(T)/ρ*Cp    -   with ρ and Cp the volume mass and the specific heat.

By discretising this equation with the help of appropriate software andby using the values for thermal conductivity given by a model called theJeffrey model, a family of curves is obtained which constitute athermogram.

The thermal conductivity can be determined by using the thermogram whichis constituted on the basis of the only experimental data available. Inthis regard, it is advisable to rewrite the heat equation by bringingout two temperature dependant coefficients:$\frac{\partial T}{\partial t} = {{a\frac{\partial^{2}T}{\partial x^{2}}} + {b\left( \frac{\partial T}{\partial x} \right)}^{2}}$

-   -   in which:        ${a = \frac{k}{\rho\quad C_{F}}},{b = {{\frac{1}{k} \cdot \frac{\mathbb{d}k}{\mathbb{d}T}}a}}$

By writing this equation twice for two very close locations, the firstat the point x and the second at the point x+dx, a system of twoequations in two unknowns is obtained. It is assumed that thecoefficients a and b at points x and x+dx are equal. By putting thissystem into matrix form, it can be solved very simply by means ofappropriate software, and the thermal conductivity of the sample can befound.

The phase change materials currently called PCMs (Phase Change Material)are alkane polymers with a solid-liquid phase change temperature varyingbetween 0 and 65° C. The PCMs offer an advantage for static uses, forexample, storage, and dynamic uses, for example, the transport ofthermal energy.

The addition of microcapsules (10 μm to 1,000 μm) of PCM materials suchas for example, naphthalene in the solid phase in a liquid in suspensiongives a biphasic mixture in liquid form currently called <<PCMS)> whichcan be put into circulation by use of conventional methods, for example,a pump. This aqueous solution allows the combining in an ecological andeconomical manner of the advantages of storage and distribution ofenergy in the form of heat and cold, and of indirect systems.

Such a PCMS is constituted by the ice slurry. The addition of smallgrains or flakes of ice into an aqueous solution yields a mixture in theliquid form which can be pumped. This mixture offers the possibility ofcombining in an ecological and economical manner the advantages ofstoring of cold and of indirect cooling with the high powerrefrigerating of direct expansion.

With respect to probe 50 in particular, other methods of constructioncan be envisaged. The sensors for temperature and the measurement of aconductivity are available on the market. Their arrangement on animmersion support in the multi-functional fluid could be adapted as afunction of requirements and applications.

1-10. (canceled)
 11. A method for continuous measurement of thermalconductivity of a multi-functional fluid, the method comprising thesteps of: passing a sample of the multi-functional fluid through a spacedelimited by a first input face and a second exit face; generating anincrease in temperature of the sample of multi-functional fluid, atleast by a very brief impulse of heat flux transmitted to the sample,through the first input face; measuring the temperature increase in atleast three separated points within the sample; determining with thetemperature increase measurement, an evolution of the multi-functionalfluid temperature at the three points as a function of time; determiningthermodynamic characteristics of the sample of the multi-functionalfluid as a function of the evolution; and calculating a thermalconductivity of the sample.
 12. The method according to claim 11,further comprising the step of transmitting the impulses of heat flux ina repetitive manner; and establishing a thermogram consisting oftemperature evolution curves as a function of an amount of time betweenthe transmitting the impulses of heat flux through the first input faceand the evolution of temperature as determined at the three separatedpoints within the sample.
 13. The method according to claim 11, furthercomprising the step of deducing the thermal conductivity with thefollowing equation:${\frac{\partial T}{\partial t} + {{\alpha(k)}\left\lbrack {{{\frac{1}{k} \cdot \frac{\mathbb{d}k}{\mathbb{d}T}}\left( \frac{\partial T}{\partial x} \right)^{2}} + \frac{\partial^{2}T}{\partial x^{2}}} \right\rbrack}} = 0$where: T is the temperature; k is the thermal conductivity dependentupon the temperature; t is the time; a is the thermal diffusivitydependant upon k and which is equal to: k(T)/ρ-Cp with p and Cp beingthe volume mass and the specific heat.
 14. A device for continuousmeasurement of thermal conductivity of a multi-functional fluid, thedevice comprising; a means designed to pass a sample of themulti-functional fluid through a space delimited by a first input faceand a second exit face of the sample; a means for heating the sample tovary a temperature of the sample, a means to measure variation of thetemperature of the sample a means to transmit to the sample, at least avery brief impulse of heat flux, through the first input face, a meansto measure a heat wave at three or more separate points within thesample; a means to determine, on a basis of values measured, atemperature evolution of the multi-functional fluid as a function oftime at the separate points within the sample; a means to deduce, fromthe temperature evolution, thermodynamic characteristics of the sampleof the multi-functional fluid; and a means to calculate thermalconductivity of this sample; the device for continuously measuring thethermal conductivity of the multi-functional fluid comprising the stepsof: passing the sample of the multi-functional fluid through the spacedelimited by the first input face and the second exit face; generatingthe increase in temperature of the sample of the multi-functional fluid,at least by the very brief impulse of heat flux transmitted to thesample, through the first input face; measuring the temperature increasein the at least three separated points within the sample; determiningwith the temperature increase measurement, the evolution of themulti-functional fluid temperature at the three points as a function oftime; determining the thermodynamic characteristics of the sample of thesaid multi-functional fluid as a function of the evolution; andcalculating the thermal conductivity of the sample.
 15. The deviceaccording to claim 14, wherein the means to pass the sample of themulti-functional fluid through the space delimited by the first andsecond faces includes an enclosure (31) with an insulating lining (32)and an interior coating of polished metal (33), which is continuouslytraversed by the multi-functional fluid.
 16. The device according toclaim 14, wherein the means (37) to transmit the at least one very briefimpulse of the heat flux comprises at least one laser (40).
 17. Deviceaccording to claim 14, wherein the means to transmit the at least onevery brief impulse of the heat flux comprises an emitter tube (21). 18.The device according to claim 14, wherein the means to measure the heatwave which has traversed the sample comprises a receiver tube (22). 19.The device according to claim 14, wherein the means to determine thetemperature evolution of the multi-functional fluid as a function oftime comprises at least three temperature probes (S1, S2, S3) designedto measure the temperature of the sample of the multi-functional fluidat the at least three separated points within the sample.
 20. The deviceaccording to claim 14, wherein the means to deduce, from the temperatureevolution at the three separate points in the sample of multi-functionalfluid, the thermodynamic characteristics of the sample and to calculatethe thermal conductivity comprises an arithmetic unit designed toreceive from the temperature probes (S1, S2, S3), the signalscorresponding to the values measured.